Test self referencing 'file'

When specifying the 'file' key to a value of it's own name, test
that this works and does not cause a false positive circular reference
error.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

tests/fixtures/extends/specify-file-as-self.yml

@@ -0,0 +1,16 @@
+myweb:
+  extends:
+    file: specify-file-as-self.yml
+    service: web
+  environment:
+    - "BAR=1"
+web:
+  extends:
+    file: specify-file-as-self.yml
+    service: otherweb
+  image: busybox
+  environment:
+    - "BAZ=3"
+otherweb:
+  environment:
+    - "YEP=1"